All three types of dinosaurs had life spans of about 25 to 30 years. But they didn't reach full adult size until age 20 to 25. Waiting until they were fully grown to reproduce would have been risky, according to Werning.

"It makes a lot of sense that [dinosaurs] wouldn't have the same strategy as birds," she said.

"What Sarah and Andrew have done in their paper is find exactly the same thing but in a different group of dinosaurs and by using a different marker of reproductive maturity," said Kristi Curry Rogers, a paleontologist at Macalester College in St. Paul, Minnesota.

Curry Rogers is a co-author of the 2007 birdlike-dino study with Gregory Erickson, a paleontologist at Florida State University.

She said both studies highlight the uniqueness of sexual maturity in birds.

"[Bird] evolution is a very fascinating mosaic of characteristics that deserves a lot more study," she said. "We're still at the beginning of teasing apart all the fine details."